Emergency Signalling in an IP Multimedia Subsystem Network

ABSTRACT

A method and apparatus for handling emergency signalling in an IP Multimedia Subsystem network. A Proxy-Call Session Control Function receives a message requesting set up of a session. The message is associated with an IP Multimedia Private Identity. The P-CSCF determines that the message relates to an emergency call. If an emergency override tag associated with the IMPI is provisioned or set at the P-CSCF, then the message is forwarding the SIP message to a Serving-Call Session Control Function (S-CSCF) for further handling. However, if an emergency override tag associated with the IMPI is not provisioned or set at the P-CSCF, the message is forwarded to an Emergency-Call Session Control Function (E-CSCF). The invention allows the P-CSCF to exercise control over the handling of emergency signalling.

PRIORITY CLAIM

This application is a continuation application of patented U.S. patentapplication Ser. No. 13/505,336 filed on May 1, 2012 (U.S. Pat. No.8,996,673 to be issued Mar. 31, 2015), which is a U.S. NationalApplication filed under 35 U.S.C. 371 of International Application No.PCT/EP2009/064463 filed on Nov. 2, 2009. The contents of this documentare hereby incorporated by reference herein

TECHNICAL FIELD

The invention relates to the field of emergency signalling in an IPMultimedia Subsystem network.

BACKGROUND

The IP Multimedia Subsystem (IMS) is the technology defined by the ThirdGeneration Partnership Project (3GPP) to provide IP Multimedia servicesover mobile communication networks. IP Multimedia services provide adynamic combination of voice, video, messaging, data, etc. within thesame session.

The IMS makes use of the Session Initiation Protocol (SIP) to set up andcontrol calls or sessions between user terminals. The SessionDescription Protocol (SDP), carried by SIP signals, is used to describeand negotiate the media components of the session. Whilst SIP wascreated as a user-to-user protocol, the IMS allows operators and serviceproviders to control user access to services and to charge usersaccordingly.

FIG. 1 illustrates schematically how the IMS fits into the mobilenetwork architecture in the case of a General Packet Radio Service(GPRS) access network. As shown in FIG. 1 a control of communicationsoccurs at three layers (or planes). The lowest layer is the ConnectivityLayer 1, also referred to as the bearer plane and through which signalsare directed to/from user equipment (UE) accessing the network. Theentities within the connectivity layer 1 that connect an IMS subscriberto IMS services form a network that is referred to as theIP-Connectivity Access Network, IP-CAN. The GPRS network includesvarious GPRS Support Nodes (GSNs). A gateway GPRS support node (GGSN) 2acts as an interface between the GPRS backbone network and othernetworks (radio network and the IMS network). The middle layer is theControl Layer 4, and at the top is the Application Layer 6.

The IMS 3 includes a core network 3 a, which operates over the middle,Control Layer 4 and the Connectivity Layer 1, and a Service Network 3 b.The IMS core network 3 a includes nodes that send/receive signalsto/from the GPRS network via the GGSN 2 a at the Connectivity Layer 1and network nodes that include Call/Session Control Functions (CSCFs) 5,which operate as SIP proxies within the IMS in the middle, Control Layer4. The 3GPP architecture defines three types of CSCFs: the Proxy CSCF(P-CSCF) which is the first point of contact within the IMS for a SIPterminal; the Serving CSCF (S-CSCF) which provides services to the userthat the user is subscribed to; and the Interrogating CSCF (I-CSCF)whose role is to identify the correct S-CSCF and to forward to thatS-CSCF a request received from a SIP terminal via a P-CSCF. The top,Application Layer 6 includes the IMS service network 3 b. ApplicationServers (ASs) 7 are provided for implementing IMS service functionality.

When an emergency call is placed using a UE, it is normally routed fromthe P-CSCF to and Emergency CSCF (E-CSCF). The E-CSCF is concerned onlywith handling emergency calls. The E-CSCF must determine the mostappropriate Public Safety Answering Point (PSAP), for example thenearest emergency services call centre, and forwards emergency INVITEmessages to the PSAP. The E-CSCF is attached to a Location RetrievalFunction (LRF), which allows it to determine the location of the callingUE.

In some circumstance it may be necessary to route an emergency call viathe S-CSCF. This is the case where, for example, a security associationhas not been established between the P-CSCF and the UE at registration,and the local emergency authorities do not accept emergency calls thathave not been authenticated. This is discussed in 3GPP TSG-SA WG2S2-095075. Where there is no security association, the S-CSCF performs aSIP-digest authentication for every dialogue. The Emergency session istherefore sent via the S-CSCF, which queries the UE's credentials.Another scenario in which the emergency call might be routed via theS-CSCF is where either the S-CSCF or an AS needs to perform a furthernumber analysis. An example of this is a large corporation that hastheir own emergency response procedures. In this case, the corporationmay choose to re-route a call to a public emergency number such as 112to their own emergency centre before contacting the appropriate PSAP.

This solution to allowing a UE that has not established a securityassociation with the P-CSCF works in most network scenarios. However, itwill not work in a network that handles both fixed and mobile users,because for mobile users using such a network, direct routing is neededto the E-CSCF, in particular for those cases where the user is roaming(due, in part, to regulatory and liability issues). Furthermore, routingemergency sessions via the S-CSCF is not optimal in the case of a fixedaccess user that is already authenticated, as it requires additionalproxy nodes in the path, that during an emergency situation may become abottle neck that can become overloaded if many users are trying toaccess the emergency services (as it in general serves many more usersthan a P-CSCF).

SUMMARY

It is an object of the invention to improve the handling emergencysignalling in an IP Multimedia Subsystem network where the UserEquipment (UE) has not established a security association with the ProxyCall/Session Control Function (P-CSCF).

According to a first aspect of the invention, there is provided a methodof handling emergency signalling in an IP Multimedia Subsystem (IMS)network. A Proxy-Call Session Control Function (P-CSCF) receives amessage requesting set up of a session. The message is associated withan IP Multimedia Private Identity (IMPI). The P-CSCF determines that themessage relates to an emergency call. If an emergency override tagassociated with the IMPI is provisioned or set at the P-CSCF, then themessage is forwarding the SIP message to a Serving-Call Session ControlFunction (S-CSCF) for further handling. However, if an emergencyoverride tag associated with the IMPI is not provisioned or set at theP-CSCF, the message is forwarded to an Emergency-Call Session ControlFunction (E-CSCF). The invention allows the P-CSCF to exercise a degreeof control over the handling of emergency signalling, rather thanautomatically forwarding the signalling to a S-CSCF in the event thatthe UE does not have an established security association with theP-CSCF.

As an option, the emergency override tag is provisioned or set in theevent that the IMPI is associated with one of a predetermined accessnetwork, a predetermined access network type (for example, fixed-lineaccess network), a predetermined authentication method (for example,Digest authentication), and a User Equipment that is not located in avisited network.

As a further option, the emergency override tag for the IMPI is receivedfrom the S-CSCF in one of SIP 200 OK message and a SIP NOTIFY message.

According to a second aspect of the invention, there is provided aP-CSCF for use in an IMS network. A receiver is provided for receiving amessage associated with an IMPI, the message requesting set up of asession. A determining function is provided for determining that themessage relates to an emergency call, and an emergency override functionis provided for determining that an emergency override tag associatedwith the IMPI is provisioned or set at the P-CSCF. A transmitter is alsoprovided. In the event that it is determined that an emergency overridetag associated with the IMPI is provisioned or set at the P-CSCF, thetransmitter is arranged to forward the message to a S-CSCF to be furtherhandled. In the event that the emergency override tag associated withthe IMPI is not provisioned or set at the P-CSCF, the transmitter isarranged to forward the message to an E-CSCF.

The receiver is optionally arranged to receive an emergency override tagfor the IMPI registered from a S-CSCF in one of SIP 200 OK message and aSIP NOTIFY message.

According to a third aspect of the invention, there is provided a S-CSCFfor use in an IMS network. A first receiver is provided for receiving aregistration request from a P-CSCF, the registration request beingassociated with an IMPI. A first transmitter is provided for sending aquery to a Home Subscriber Server (HSS). The query is a request for anemergency override tag associated with the IMPI and stored at the HSS. Asecond receiver is provided for receiving a response to the query, theresponse including the emergency override tag associated with the IMPI.A second transmitter is provided for sending a message to the P-CSCF,the message including the emergency override tag associated with theIMPI.

The message sent to the P-CSCF is optionally sent in response to one ofthe registration request and a user data subscription request.

According to a fourth aspect of the invention, there is provided a HSSfor use in an IMS network. The HSS is provided with a memory is forstoring an emergency override tag associated with an IMPI, the emergencyoverride tag for use by a P-CSCF in determining where to route emergencysignalling. A receiver is also provided for receiving from a S-CSCF aquery requesting the emergency override tag. A transmitter is providedfor sending a response to the S-CSCF, the response including theemergency override tag.

According to a fifth aspect of the invention, there is provided acomputer program comprising computer readable code which, when run on aP-CSCF, causes the P-CSCF to behave as a P-CSCF as described above inthe second aspect of the invention.

According to a sixth aspect of the invention, there is provided acomputer program comprising computer readable code which, when run on aS-CSCF, causes the S-CSCF to behave as a S-CSCF as described above inthe third aspect of the invention.

According to a seventh aspect of the invention, there is provided acomputer program comprising computer readable code which, when run on aHSS, causes the HSS to behave as a HSS as described above in the fourthaspect of the invention.

According to an eighth aspect of the invention, there is provided acomputer program product comprising a computer readable medium and acomputer program as described above in any of the fifth, sixth orseventh aspects of the invention, wherein the computer program is storedon the computer readable medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically in a block diagram an IMS network inassociation with a mobile network architecture of a General Packet RadioService (GPRS) access network;

FIG. 2 is a signalling diagram showing the provisioning of a HomeSubscriber Server with a P-CSCF emergency override tag according to anembodiment of the invention.

FIG. 3 illustrates schematically in a block diagram an example usersubscription with an emergency override tag associated with a privateidentity;

FIG. 4 is a signalling diagram illustrating a registration processaccording to an embodiment of the invention;

FIG. 5 is a signalling diagram illustrating a call initiation processaccording to an embodiment of the invention;

FIG. 6 illustrates schematically in a block diagram a Proxy Call SessionControl Function according to an embodiment of the invention; and

FIG. 7 illustrates schematically in a block diagram a Serving CallSession Control Function according to an embodiment of the invention;and

FIG. 8 illustrates schematically in a block diagram a Home SubscriberServer according to an embodiment of the invention.

DETAILED DESCRIPTION

An indicator is associated with a user account, and provisioned in theHome Subscriber Server (HSS). The indicator is subsequently provided toa Proxy-Call Session Control Function (P-CSCF) for use by the P-CSCF indetermining where to route emergency signalling. The indicator isreferred to herein as an emergency override tag. Referring to FIG. 2, aprovisioning system 8 sends (S1) a message to a HSS 9 for provisioningthe emergency override tag. The HSS 9 responds (S2) with anacknowledgement.

Referring to FIG. 3, the emergency override tag is this example isassociated with IP Multimedia Private Identity (IMPI) 1 provisioned inthe HSS 9. The emergency override tag need not be associated with onlythe Private User Identity 1, but may additionally be associated with atype of authentication method used. For example, if IMPI 1 isprovisioned with credentials for both Digest authentication and IMSAuthentication and Key Agreement (AKA), the emergency override tag maybe set to only be used where the Digest authentication method is used.

Referring to FIG. 4, there is illustrated a User Equipment (UE) 10, aP-CSCF 11, a Serving-Call Session Control Function (S-CSCF) 12, a HSS 9and an Application Server 13. During registration of the UE 10, thefollowing steps, with the following numbering corresponding to thenumbering of FIG. 3:

S3. The UE 10 sends a Register request to the P-CSCF 11. This is thesame for both emergency registration and non-emergency registration.

S4. The P-CSCF forwards the Register request to the S-CSCF 12.

S5. The S-CSCF sends a query to the HSS 9 associated with the user ofthe UE 10 over the CX interface.

S6. The HSS 9 responds to the query over the Cx interface, the responseincluding the emergency override tag. The emergency override tag is thenstored at the S-CSCF for the duration of the user's registration.

The emergency override tag can then be sent to the P-CSCF 11 either inthe Registration response (200 OK) message (see steps S7 and S9) or aspart of a notification message sent in the event that the P-CSCF 11subscribes to user data from the S-CSCF 12 (see steps S10 and S11).

S7. In the first alternative, the S-CSCF 12 adds the emergency overridetag to a SIP 200 OK message, and sends the SIP 200 OK message to theP-CSCF 11 in response to the Register message (sent in step S4). TheP-CSCF then stores the emergency override tag, and removes it from theSIP 200 OK message. The process continues at step S9.

S8. In the second alternative, the S-CSCF 12 sends a SIP 200 OK messageto the P-CSCF 11 in response to the Register message (sent in step S4).

S9. The SIP 200 OK message is sent from the P-CSCF 11 to the UE 10.

S10. In the second alternative, the P-CSCF 11 sends a Subscribe requestto the S-CSCF 12 to subscribe to user data.

S11. As the S-CSCF 12 has stored the emergency override tag, it includesthis in a Notify message and sends the notify message to the P-CSCF 11.

Whether the first or the second alternative is used, the P-CSCF becomesprovisioned with the emergency override tag.

The Cx interface between the S-CSCF 12 and the HSS 9 must be extended inorder to allow the S-CSCF 12 to request the emergency override tag fromthe HSS 9. For example, a new attribute/value pair (AVP) may beprovided. An example formal of a suitable AVP is as follows:

AVP Format

-   -   Emergency-Data-Item::=<AVP Header:        123445567>[Emergency-override]

Alternatively, instead of introducing a new AVP, the Emergency-overrideparameter is included in an existing AVP such as a User Data AVP.

When a call is initiated by the UE, and the call is directed to a URIthat may be IMS emergency related, e.g., 112, the P-CSCF must determinewhether or not to send the call to the S-CSCF 12 or an E-CSCF 14. Callsignalling is illustrated in FIG. 5, with the following numberingcorresponding to the numbering of FIG. 5:

S12. The UE 10 sends a SIP INVITE to set up the emergency call to theP-CSCF 11.

S13. The P-CSCF 11 determines whether the SIP INVITE is related to anemergency call. This may be done by, for example, provisioning theP-CSCF with URIs and numbers associated with emergency services. If thecall is an emergency call, then the P-CSCF determines whether theemergency override tag is present or set. If it is not present, or notset, then the process continues at step S18. It the emergency overridetag associated with the IMPI is present and set, then the processcontinues at step S14.

S14. The SIP INVITE is forwarded from the P-CSCF 11 to the S-CSCF 12.The S-CSCF 12 can then perform a SIP-Digest authentication if required.This step would then result in a challenge towards the user (not shown),before continuing with following steps.

S15. If further number analysis is required by an AS to determine if thecall is an emergency call, for instance in the case of an enterpriseservice, the SIP INVITE is forwarded from the S-CSCF 12 to an AS 13.

S16. The AS 13 makes a routing decision on whether the number is anemergency number that should be routed to the emergency centre. The AS13 decides that this is the case, and forwards the request onwards tothe S-CSCF 12.

S17. The SIP INVITE is then routed from the S-CSCF 12 to the E-CSCF oncethe SIP-Digest authentication procedure is complete or the AS invocationis completed. The process continues at step S19.

S18. As the emergency override tag is not present or not set, the P-CSCFforwards the SIP INVITE to the E-CSCF 14.

S19. The E-CSCF selects a PSAP (not shown) and forwards the SIP INVITEtowards the selected PSAP.

The invention allows an HSS to be provisioned with an emergency overridetag associated with an IMPI. This can be sent to the P-CSCF to help theP-CSCF to make a decision regarding routing an emergency call directlyto an E-CSCF, or routing an emergency call via an S-CSCF. This isparticularly useful when the UE is located in a network that handlesboth fixed and mobile access. Such a network, using prior art methods,would route all emergency calls via an S-CSCF, but this is unnecessarywhen the UE is using an authenticated fixed-line access network.Furthermore, routing emergency calls via an S-CSCF is not desirable whena mobile UE is roaming in a network other than the home network at whichthe S-CSCF is located, as an emergency call should ideally be routedlocally in that country to prevent network failures in another (home)country from affecting local emergency calls. The emergency override tagcan be set to ensure that an emergency call is not routed via an S-CSCFin the event that the IMPI is associated with a fixed line access, orthe UE is not in its home network.

Referring now to FIG. 6, a P-CSCF 11 is provided with a receiver 15 forreceiving a session set up message such as a SIP INVITE, as described instep S12. The receiver 15 may also be arranged to receive the emergencyoverride tag from S-CSCF 12 in either a SIP 200 OK message, or a SIPNOTIFY message, as described in steps S7 and S11 respectively. Ofcourse, this may be performed by a different receiver (not shown) at theP-CSCF 11.

An emergency call determining function 16 is provided for determiningthat the session set up message relates to an emergency call, forexample by comparing a URI in the session set up message with a list ofknown URIs that relate to emergency services.

An emergency override function 17 is provided for determining that anemergency override tag 18 associated with the IP Multimedia PrivateIdentity is provisioned or set at the P-CSCF 11. The emergency overridetag 18 is typically stored in a memory 19.

A transmitter 20 is provided for forwarding the session set up messageto either the S-CSCF 12 in the event that the emergency override tag 18is provisioned and set, or the E-CSCF 14 in the event that the emergencyoverride tag 18 is either not provisioned or not set.

It will be appreciated that different configurations of receiver andtransmitter can be used. For example, all the receiver and transmitterdescribed above may be implemented in a single transceiver.

The above description of the P-CSCF 11 assumes a hardware implementationof the invention, although the skilled person will appreciate thatcertain elements, for example, the emergency call determining function16 and the emergency override function 17 may be implemented usingsoftware. In this case the memory 19 is a computer readable medium inwhich a computer program 21 is stored, and a processor 22 is providedfor running the computer program.

Referring now to FIG. 7, a S-CSCF 12 is provided with a first receiver23 for receiving a Register request from the P-CSCF 11 as described instep S4. A first transmitter 24 is provided for sending a Cx query tothe HSS 9, as described in step S5. A second receiver 25 is provided forreceiving the Cx response as described in step S6. A second transmitter26 is also provided for sending a message to the P-CSCF 11, the messageincluding the emergency override tag 18. The message may be sent to theP-CSCF 11 in response to one of the registration request or a user datamay be also provided for storing the emergency override tag 18. Aprocessor 27 is also provided for handling the signalling, and a memory28 may be provided for storing the emergency override tag 18.

It will be appreciated that different configurations of receiver andtransmitter can be used. For example, all of the receivers andtransmitters described above may be implemented in a single transceiver.

The above description of the S-CSCF 12 assumes a hardware implementationof the invention, although the skilled person will appreciate thatcertain elements, such as instructions for handling the signalling, maybe implemented using software. In this case the memory 28 is a computerreadable medium in which a computer program 29 is stored, which can berun using the processor 27.

FIG. 8 illustrates a HSS 9, which is provided with a memory 30 forstoring an emergency override tag 18 that is associated with an IMPI 31.Of course, the memory is likely to store IMPIs and emergency overridetags for many users. A receiver 32 is provided for receiving a Cx queryfrom the S-CSCF 12, as described in step S5, and a transmitter 33 isprovided for responding to the Cx query with the required tag 18. Aprocessor 34 is provided for handling the signalling and obtaining therequired information from the memory 30.

The above description of the HSS 9 assumes a hardware implementation ofthe invention, although the skilled person will appreciate that certainelements may be implemented using software. In this case the memory 30is a computer readable medium in which a computer program 35 is stored,which can be run using the processor 34. The skilled person willappreciate that different information may be stored in differentphysical memories at the HSS 9.

It will be appreciated by the person of skill in the art that variousmodifications may be made to the above-described embodiments withoutdeparting from the scope of the invention as defined in the appendedclaims.

The following acronyms have been used in this description:

-   3GPP Third Generation Partnership Project-   AS Application Server-   AVP Attribute-value pair-   E-CSCF Emergency-Call Session Control Function-   GGSN Gateway GPRS support node-   GPRS General Packet Radio Service-   GSNs GPRS Support Node-   HSS Home Subscriber Server-   I-CSCF Interrogating-Call Session Control Function-   IMPI IP Multimedia Private Identity-   IMS IP Multimedia Subsystem-   LRF Location Retrieval Function-   P-CSCF Proxy-Call Session Control Function-   PSAP Public Safety Answering Point-   S-CSCF Serving-Call Session Control Function-   SDP Session Description Protocol-   SIP Session Initiation Protocol-   UE User Equipment

1. A method of handling emergency signaling in an Internet Protocol (IP)Multimedia Subsystem (IMS) network, the method comprising: at aProxy-Call Session Control Function (P-CSCF), receiving a messageassociated with an IP Multimedia Private Identity, the messagerequesting a setup of a session; at the P-CSCF, determining that themessage relates to an emergency call; in an event that an emergencyoverride tag associated with the IP Multimedia Private Identity isprovisioned at the P-CSCF, forwarding the message to a Serving-CallSession Control Function (S-CSCF), wherein the S-CSCF handles themessage and the emergency call; and in an event that an emergencyoverride tag associated with the IP Multimedia Private Identity is notprovisioned at the P-CSCF, forwarding the message to an Emergency-CallSession Control Function (E-CSCF), wherein the E-CSCF handles themessage and the emergency call.
 2. The method according to claim 1,wherein the emergency override tag is provisioned in an event that theIP Multimedia Private Identity is associated with one of a predeterminedaccess network, a predetermined access network type, a predeterminedauthentication method, and a User Equipment that is not located in avisited network.
 3. The method according to claim 1, further comprising:prior to receiving the SIP message requesting setup of a session,receiving an emergency override tag for the IP Multimedia PrivateIdentity registered from the S-CSCF in one of a SIP 200 OK message and aSIP NOTIFY message.
 4. A Proxy-Call Session Control Function (P-CSCF)node for use in an Internet Protocol (IP) Multimedia Subsystem (IMS)network, the P-CSCF node comprising: a receiver configured for receivinga message associated with an IP Multimedia Private Identity, the messagerequesting setup of a session; at least one microprocessor; anon-transitory computer-readable storage medium, coupled to the at leastone microprocessor, comprising computer-readable instructions, whenexecuted by the at least one microprocessor, are configured for:determining that the message relates to an emergency call, anddetermining if an emergency override tag associated with the IPMultimedia Private Identity is provisioned at the P-CSCF node; and atransmitter configured for, in an event that an emergency override tagassociated with the IP Multimedia Private Identity is provisioned at theP-CSCF, forwarding the message to a Serving-Call Session ControlFunction (S-CSCF), wherein the S-CSCF handles the message and theemergency call, and in an event that an emergency override tagassociated with the IP Multimedia Private Identity is not provisioned atthe P-CSCF, forwarding the message to an Emergency-Call Session ControlFunction (E-CSCF), wherein the E-CSCF handles the message and theemergency call.
 5. The P-CSCF node according to claim 4, wherein thereceiver is configured for receiving an emergency override tag for theIP Multimedia Private Identity registered from a S-CSCF in one of a SIP200 OK message and a SIP NOTIFY message.
 6. A Serving-Call SessionControl Function (S-CSCF) node for use in an Internet Protocol (IP)Multimedia Subsystem (IMS) network, the S-CSCF node comprising: at leastone microprocessor; a non-transitory computer-readable storage medium,coupled to at least one microprocessor, comprising computer-readableinstructions, when executed by the at least one microprocessor, areconfigured for: receiving a registration request form a Proxy-CallSession Control Function (P-CSCF), the registration request beingassociated with an IP Multimedia Private Identity, sending a query to aHome Subscriber Server, the query requesting an emergency override tagindicator associated with the IP Multimedia Private Identity and storageat the Home Subscriber Server, receiving a response to the query, theresponse including the emergency override tag indicator associated withthe IP Multimedia Private Identity, and sending to the P-CSCF a message,the message including the emergency override tag indicator associatedwith the IP Multimedia Private Identity; and wherein, in an event thatan emergency override tag associated with the IP Multimedia PrivateIdentity is sent to the P-CSCF, the S-CSCF handles the message and anemergency call, and in an event that an emergency override tagassociated with the IP Multimedia Private Identity is not sent to theP-CSCF, the Emergency-Call Session Control Function (E-CSCF) handles themessage and an emergency call.
 7. The S-CSCF node according to claim 6,wherein the message sent to the P-CSCF is sent in response to one of theregistration request and a user data subscription request.
 8. A HomeSubscriber Server for use in an IP Multimedia Subsystem (IMS) network,the Home Subscriber Server comprising: at least one microprocessor; anda non-transitory computer-readable storage medium, coupled to the atleast one microprocessor, comprising computer-readable instructions,when executed by the at least one microprocessor, are configured for:storing an emergency override tag indicator associated with an IPMultimedia Private Identity, the emergency override tag indicator foruse by a Proxy-Call Session Control Function in determining where toroute emergency signaling, receiving from a Serving-Call Session ControlFunction a query requesting the emergency override tag indicator, andsending to the Serving-Call Session Control Function a response, theresponse including the emergency override tag indicator, wherein in anevent that an emergency override tag associated with the IP MultimediaPrivate Identity is provisioned at the P-CSCF, forwarding the SIPmessage to a Serving-Call Session Control Function (S-CSCF), wherein theS-CSCF handles an emergency call; and in an event that an emergencyoverride tag associated with the IP Multimedia Private Identity is notprovisioned at the P-CSCF, forwarding the SIP message to anEmergency-Call Session Control Function (E-CSCF), wherein the E-CSCFhandles an emergency call.